1. Field of the Invention
This invention relates to digital data transmission systems. In particular, the present invention relates to a transceiver for echo and near-end crosstalk (NEXT) cancellation without a loop timing configuration and a method of the transceiver for echo and NEXT cancellation.
2. Description of the Related Art
Many communication applications, such as high speed data communication devices, require high insulation against noise, and must meet high precision requirements necessary for digital transmission/reception, e.g., a transceiver implemented in Gigabit Ethernet standard. In such applications, communications over an unshielded twister pair (UTP) or other communications loop may require a very low or even error free transmission of coded data. For example, a bit error rate (BER) equal to or less than 10−10 is required for use in the Gigabit Ethernet. Such bit error rates are difficult to obtain given unknown delays, attenuation, dispersion, noise, and inter-symbol interference (ISI) introduced by and/or on the communications channel. One area where BER must be closely controlled is at the transceiver adaptive echo canceller in duplex digital data transmission systems.
Furthermore, echo and near-end crosstalk are usually serious problems in digital data transmission systems. The schematic diagram in FIG. 1 illustrates near-end crosstalk and echo problems. The UTPs shown in FIG. 1 are twisted pairs 10 and 12 following category 5 standard. The twisted pairs 10 and 12 are transmission media. The Hybrid devices 10a, 10b, 12a, and 12b combine transmitted and received signals. The data are transmitted to channel through the hybrid devices 10a, 10b, 12a, and 12b. In the embodiment, the transmitting ends Tx1 and Tx2 and the receiving ends, Rx1 and Rx2 are at the local end. The transmitting end Tx and the receiving end Rx are at the remote end. When the signal is transmitted to the hybrid device 10a, a portion of the signal will be received by the local receiving end Rx1, and which is called “echo”. If the receiving end Rx1 receives a signal from other channels in the local end, i.e. the signal is transmitted through the twisted pair 12, the signal received by the receiving end Rx1 is referred to as near-end crosstalk.
Conventional techniques for echo and near-end crosstalk cancellation generally use an adaptive canceller to estimate impulse responses of echo and near-end crosstalk and subtract them from the received signal. FIG. 2 is a schematic diagram illustrating the configuration of a typical transceiver comprising an echo canceller and a near-end crosstalk (NEXT) canceller. A hybrid device 208, connected to the channel (not shown in FIG. 2), combines transmitted data with received data and transmits the transmitted data to a channel. A transmitting end Tx1 and a receiving end Rx1 are at the same twisted pair of a local end. A transmitting end Tx2 is at another twisted pair of the local end. The digital-to-analog converter (DAC) 204 converts a digital signal to an analog signal. The transmitting end Tx1 transmits the analog signal from the hybrid device 208 to the channel. The receiving end Rx1 receives a signal from the channel through the hybrid deice 208. The received signal is transmitted to an analog-to-digital converter (ADC) 210. The ADC 210 digitizes the received signal and feeds a digital signal to an equalizer 212. A timing recovery circuit 214 receives an equalized signal from the equalizer 212 and recovers a clock from the equalized signal. The echo canceller 202 is coupled between the transmitting end Tx1 and the receiving end Rx1. The echo canceller 202 estimates impulse response of echo. Then, the echo is removed from an equalized signal by an adder 220. The NEXT canceller 216 is coupled between the transmitting end Tx2 and the receiving end Rx1. The NEXT canceller 216 estimates impulse response of near-end crosstalk. Then, the near-end crosstalk is removed from the equalized signal by an adder 220. Typically, the clock in the transmitting end Tx1 and Tx2 is produced by a local oscillator. The clock in the receiving end Rx1 is recovered by the received signal. Thus, the clock domain in the transmitting end is different from that in the receiving end. That will cause asynchronous problems in circuit design.
To solve the above problem, a loop timing system is provided. FIG. 3 is a schematic diagram illustrating the configuration of the typical loop timing system. The loop timing system is applied in a Gigabit Ethernet communication system. Four twisted pairs in the loop timing system transmit the data. In the loop timing system, a transmitting end Tx1m and a receiving end Rx1m are at the master end 302. A transmitting end Tx2s and a receiving end Rx2s are at the slave end 304. A clock of the transmitting end Tx1m is produced by a local oscillator of the master end 302. A clock in the transmitting end Tx2s is recovered by a signal received by the receiving end Rx2s. Thus, at the slave end 304, the clock in the transmitting end Tx2s is the same as the clock in the receiving end Rx2s. After the master end 302 receives a signal transmitted from the transmitting end Tx2s, the clock in the receiving end Rx1m is recovered by the signal. Then, at the master end 302, the clock in the transmitting end Tx1m is synchronized to the clock in the receiving end Rx1m. After overcoming the problem of different clocks between the transmitting end and the receiving end, echo and near-end crosstalk cancellation are accomplished completely. However, because the loop timing system is much more complex than other communication systems, the development cost of the system is high and such a complex design is not suitable in some digital data communication systems.